The importance of synapses in learning and memory has been elucidated in many studies. Indeed, synapse loss is thought to be one of the main contributors to the cognitive decline seen in Alzheimer's disease (AD). Apolipoprotein E receptor 2 (ApoEr2) is present at the synapse and is known to be involved in long-term potentiation and learning and memory. The correlation between Apolipoprotein E (ApoE) and risk for AD implicate important connections between ApoEr2, learning and memory, and the synapse loss seen in AD. However, the molecular mechanisms of ApoEr2 in synapse formation and loss are still unknown. We have recently identified interactions between ApoEr2 and two key molecules involved in synapse regulation: X11 and PSD-95, via the intracellular domains of ApoEr2. In addition, ApoE are ligands that affect the trafficking and proteolysis of ApoEr2. In view of the well established roles of PSD-95 and ApoE in synaptogenesis, we hypothesize that in the normal brain, ApoEr2 is involved in the formation of synapses and the architecture of dendritic spines. In support of this idea, we found that ApoEr2 is expressed in the postsynaptic density and highly expressed during the peak of synaptogenesis. Furthermore, we found that when ApoEr2 is expressed in COS7 cells and co-cultured with primary hippocampal neurons, ApoEr2 on the COS7 cell surface recruits and colocalizes with presynaptic specializations on contacting neuronal dendrites. We also tested the effects of ApoE isoforms on dendritic spine formation and we found that ApoE3 increased spine number and ApoE4 decreased spine number compared to green fluorescence protein (GFP) alone. These results suggest that ApoE is capable of inducing synapse formation, a process that we propose depends on the physical synaptic associations between ApoE, ApoEr2 and cytoplasmic adaptor proteins. We will utilize molecular/biochemical and cell biology approaches to determine the mechanism by which ApoEr2 coordinates with X11 or PSD-95 to regulate synapse and spine formation. In Aim 1, we will elucidate the functional role of the ApoEr2 on synapse formation. In Aim 2, we will determine the role of ligands (e.g., ApoE isoforms) for ApoEr2 in regulating synapse formation and ApoEr2 interactions. The proposed experiments are important for understanding the physiological actions of ApoEr2 in normal tissue and are likely to have a broad impact on the fields of synapse development and learning and memory, which are intimately involved with the formation and elimination of synapses. Furthermore, this proposal is highly relevant to public health because understanding the function, and dysfunction, of ApoEr2 at synapses could help to explain the risk of different ApoE isoforms on the synaptic loss that is a hallmark of cognitive decline in AD, potentially leading to more effective therapeutic strategies. PUBLIC HEALTH RELEVANCE: The proposed experiments are important for understanding the physiological actions of ApoEr2 in normal tissue and are likely to have a broad impact on the fields of synapse development and learning and memory, which depend on the persistent formation and elimination of synapses.